1. Field of the Invention
The present invention relates to a fixing device that is mounted in such image forming apparatus as, for instance, electrostatic copying machines, laser printers, etc. for heating and fixing toner images on paper.
2. Description of the Related Art
On fixing devices installed in image forming apparatus such as electrostatic copying machines, laser printers, etc., a halogen lamp, etc. are so far used as a heating source. This halogen lamp is installed in a hollow metallic roller and the metallic roller is heated from the inside when this halogen lamp is lighted. When a sheet of paper carrying an unfixed toner image is led to a nipping portion that is formed between this heated metallic roller and a pressuring roller pressed against this metallic roller at a specified pressure, the toner on the paper is melted and fixed on the paper.
However, existing fixing devices use a lamp as a heating source and thermal efficiency is limited to about 70%. In addition, as a lamp is arranged in the inside of a metallic roller to heat it from the inside, in order to heat the surface of the metallic roller that is used for the actual fixing operation it is necessary to keep the inside of the metallic roller at a temperature higher than the surface of the metallic roller. Because of this, there is such a demerit that an energy loss is large. Further, a long time is required to heat the inside of the metallic roller so that the surface of the metallic roller reaches a toner image fixable temperature. This long time becomes a factor to obstruct the reduction of a so-called rising time until an image forming apparatus reaches a usable state.
To solve these problems, there is a fixing device that was disclosed in the Japanese Publication of Unexamined Patent Application No. 07-295414. This fixing device uses a so-called induction heating method to generate eddy current on the surface of a heating roller comprising a magnetic material and directly heats the surface of the heating roller by resistance of the heating roller itself and the generated eddy current. However, in this induction heating method of the fixing device, the heating roller is composed of a magnetic material only and therefore, its thermal conductivity is low and the temperature on the surface of the heating roller becomes uneven along the axial direction of the heating roller.
As a result, there are such problems that a uniform fixing performance may not be maintained, the unsatisfactory fixing may be caused and the heating roller may be filmed over by a toner.
Further, due to the low thermal conductivity, there is such a problem that the obtained fixing performance may differ depending on paper size to be fixed. That is, between a relatively large size paper using the entire longitudinal direction of the heating roller and a relatively small size paper using only a part of the longitudinal direction of the heating roller, the temperature distribution generated along the longitudinal direction of heating roller becomes uneven.
Further, there is an induction heating type fixing device disclosed in the Japanese Publication of Unexamined Patent Application No. 08-76620. This induction heating type fixing device is to heat a conductive film by a magnetic field generating means and fix a toner image on a recording medium that is closely fitted to the inductive film. That is, a nip is formed by inserting a belt between the magnetic field generating means and a heating roller and a toner image on a recording medium passing through this nip is heated and fixed thereon. In this case, however, there is such a problem that as the magnetic generating means is kept in contact with the belt that is a heating element, the heat generated on the belt moves to the magnetic generating means and the heat value to be given to the recording medium decreases. Furthermore, there was also such a problem that if heat moved to the magnetic generating means, the iron loss of a coil would be caused and heating efficiency will decrease.
Further, when a paper smaller in size than the nip width was passed through the nip, a temperature difference will be produced between the passed portion and the not passed portion and there was such a problem that this temperature difference was left as a temperature hysteresis and used in the fixing of a next recording medium and an image was not uniformly fixed.